Epoxy resins are cured with various curing agents to provide cured products generally excellent in mechanical properties, water resistance, chemical resistance, heat resistance, and electrical properties, and are therefore used in various fields such as adhesives, paints, laminated plates, molding materials, and casting materials. Among such conventional epoxy resins, bisphenol A epoxy resins are most commonly used. As curing agents for epoxy resins, acid anhydrides and amine-based compounds are known, but in the field of electric and electronic parts, phenol novolac resins are often used from the viewpoint of reliability such as heat resistance. Further, various flame retardants are also used for imparting flame retardancy to cured products. As such flame retardants, bromine-containing compounds such as tetrabromobisphenol A, epoxidation products thereof, and reaction products of tetrabromobisphenol A and bisphenol A epoxy resins are generally known.
As described above, such bromine-containing compounds have excellent ability to impart flame retardancy to resins, but it has been pointed out that there is a possibility that they generate substances causing environmental pollution during disposal or incineration. In addition, there is also a concern about the toxicity of antimony compounds used as flame retardant aids. In recent years, there has been an increasing demand for halogen- and antimony-free epoxy resin compositions with the growing awareness of environmental protection. Meanwhile, epoxy resins cured using a phenol novolac resin as a curing agent are rigid and lack in flexibility while having high reliability. In recent years, there have been developed various electric and electronic parts including, in addition to conventional large packaged parts and plate-shaped parts using rigid substrates containing glass fibers as a base material, molded parts of polyimide films, PET (polyethylene terephthalate) films or flexible sheet-shaped resin substrates. Such molded parts are used for a flexible printed wiring board as a laminated structure bonded to a metal foil or a metal circuit. Such a laminated structure is usually produced by applying an epoxy resin composition in the form of varnish onto a base material such as polyimide or copper foil, removing a solvent, and curing the epoxy resin composition. In this case, a resin to be used is required to have sufficient flexibility and good adhesion to a base material such as polyimide or copper foil. In addition, from the viewpoint of ensuring the reliability of electric and electronic parts, a resin composition to be used is required to have high purity, and a cured product thereof is required to have high heat resistance.
Meanwhile, as an epoxy resin composition intended to improve the brittleness of conventional epoxy resin, Patent Document 2 discloses an epoxy resin composition containing three components, an epoxy resin, a phenol resin, and a phenolic hydroxyl group-containing aromatic polyamide resin, and describes that the epoxy resin composition has toughness and heat resistance. Further, Patent Document 3 describes that an epoxy resin composition containing an epoxy resin and a phenolic hydroxyl group-containing polyamide resin has excellent flame retardancy, and is therefore useful as a material for flexible printed wiring boards. Further, Patent Document 4 discloses a polyamide-containing varnish.
However, in order to produce such a phenolic hydroxyl group-containing polyamide resin, it is necessary to carry out a condensation reaction between a diamine component and a dicarboxylic acid component generally in the presence of a phosphorous acid compound, and therefore phosphorus-based ions such as phosphate ions and phosphite ions (hereinafter, sometimes also referred to as “ionic impurities”) remain in the resin. In a case where the resin has a relatively low molecular weight and a low viscosity, it is possible to sufficiently reduce the phosphate ions and the like by washing the resin with water. However, in a case where the molecular weight of the polyamide resin is increased, the viscosity of the polyamide resin is also increased, and therefore it becomes difficult to sufficiently reduce the phosphate ions and the like by only washing the resin with water. This may cause deterioration of electrical properties of electric and electronic parts and the like using the resin.
Patent Document 1: Japanese Patent No. 2969585
Patent Document 2: JP 2000-313787 A
Patent Document 3: WO 2004/048436
Patent Document 4: WO 02/34850